1. Field of the Invention
The present invention relates to a system and method for fabricating a pre-stressed modular construction for supporting or retaining an applied load. More particularly, the present invention relates to a system and method for pre-stressed modular retaining walls.
2. Related Art
A retaining wall is an engineered structure that has the particular task of ensuring that a given unstable, or potentially unstable, soil mass is prevented from moving under the influence of gravity. Frequently, the retaining wall is also called upon to withstand a superimposed load, a surcharge load, on and/or within the soil mass, such as a highway, together with its traffic loading, or the loading induced by the foundations of a building located in close proximity to the retaining structure. Further, the retaining wall may be required to support some other non-retaining load that is resisted by structural elements directly attached to, and/or incorporated within, the wall structure itself.
Since the early 1970""s, numerous alternative wall systems have been introduced. Examples of these systems include mechanically stabilized earth (MSE) walls and reinforced soil slopes (RSS) employing metallic or polymeric internal reinforcement; anchored walls, such as the soldier pile and lagging walls, diaphragm walls, and soil mixed walls; prefabricated modular gravity wall systems including cribs, bins, and gabions; and in-situ reinforced wall systems such as soil-nailed walls and micropile walls. However, because of the ever increasing demands that are being placed on our city and urban environments and, most noticeably, on the country""s transportation infrastructure, together With the need to preserve our natural environment while providing for the associated societal expectations, there is an increasing number of problematic sites where the currently available retaining wall options cannot provide an optimal solution. In particular, for those sites that require xe2x80x9cfoundation-upxe2x80x9d construction, there is a dearth of rapid construct, high capacity retaining wall systems possessing significant functional flexibility and which demand only a small construction footprint. Retaining structures constructed to resist soil pressures are often categorized according to their basic mechanisms of retention. The retention mechanisms include internally stabilized, externally stabilized, and hybrid systems. Alternatively, retaining walls may be categorized according to their source of support, that is, their source of equilibrating reaction forces. The sources of support for these retaining walls may be bracketed into gravity, semigravity, and nongravity.
An internally stabilized system involves reinforced soils to retain a soil mass and any surcharge loads. This reinforcing may be provided by adding reinforcement directly to the soil mass, where this augmented soil mass is providing the retaining/self-retaining structure, as the system is being constructed from the xe2x80x9cgroundxe2x80x9d up. Various types of reinforcement are available, and the soils between the layers of reinforcement are placed in a carefully controlled manner meeting design specificationsxe2x80x94that is, the placed soil is xe2x80x9cengineered fill.xe2x80x9d Frequently, pre-cast concrete elements are tied directly to these soil reinforcing components. This system forms the basic approach of Mechanically Stabilized Earth, MSE, retaining wall systems.
Alternatively, this internal stabilization via the reinforcing of the soil mass in question may proceed from the top down. In this (directionally) opposite approach, reinforcing elements are added to the existing soil mass in order to provide the existing materials with a greater degree of internal stability. As an example of this approach, the face, that is exposed as the excavation proceeds from the top down, has soil nails installed through it into the ground mass, which nails extend beyond any potential failure plane. Often, a shotcrete cover over the exposed face is placed and subsequently connected to these nails, thereby providing a protection against erosion of the soil face.
Further to the above methods of reinforcing a soil mass, driven piles or cast-in-drilled-hole piles may be used to stabilize the mass of concern. However, this approach is generally considered when the stability issue is more global in nature. By xe2x80x9cglobalxe2x80x9d is meant the situation where a body of soil is experiencing a deep-seated instability, which instability ideally needs to be eliminated.
With externally stabilized systems, a physical structure is employed to confine the body of soil. The equilibrating reaction forces, required by an externally stabilized system, are provided either through the weight of a morpho-stable structure, or by the reactions mobilized via the inclusion and/or extension of various system elements into xe2x80x9creaction zonesxe2x80x9d. The latter reactions may be generated by driving the piles of a sheet-pile wall system, for example, to sufficient depths into competent soil. Or, reactions may be generated via the use of ground anchors providing point-reactions on the externally stabilizing structure. Frequently, combinations of reaction-force-providing structural elements are employed, in a given situation, to deliver the total force equilibration required for an externally stabilized retaining wall.
With regard to sources of support, that is, with regard to the sources of the equilibrating reaction forces, retaining wall systems may be categorized into three groups. These are the groupings of (1) gravity walls, (2) semigravity walls, and (3) nongravity walls.
Gravity walls derive their capacity to resist imposed soil loads through the dead weight of the wall itself (that is the physical wall that is constructed) or through an integrated mass that can be either internally or externally stabilized. Gravity walls may be further classified into four types as follows. The first type is an internally stabilized soil mass system. Some of the examples given above are typical. The stability of a cut slope may be maintained in a top-to-bottom installation of soil nails, installed as the excavation of materials proceeds. Or, a retaining soil mass may be constructed of engineered fill, in a bottom-to-top sequence, thereby creating a soil mass possessing the required internal stability via the inclusion of reinforcing elements at regular vertical spacing. Where the soil mass is constructed from engineered fill, the face of such soil mass may be protected by using pre-cast concrete facings as with many MSE systems. Where soil nails are used, the front face is preferably protected using shotcrete or cast-in-place concrete. The second type of gravity wall is an externally stabilized soil mass system. Included in this category are simple modular pre-cast concrete walls. Such simple pre-cast concrete walls are stacked, but include no internal mechanism for enhancing structural capacity. Another example is prefabricated metal bin walls. The third type is also an externally stabilizing system. In this category are the generic walls including the masonry walls, the stone walls, xe2x80x9cdumpedxe2x80x9d (usually shaped) rock walls, and the contained rock walls, often using uniform crushed rock and known as gabion walls. The fourth system is also an externally stabilizing system. Examples are the use of cast-in-place mass concrete wall, or the cement-treated soil wall. Where the face of the treated soil wall requires protection, a pre-cast concrete panel may be used, which panel would be anchored to the treated-soil wall.
Semigravity walls derive their restraining capability through the combination of dead weight and structural resistance. Generally, these semigravity walls are externally stabilizing structures. They may be constructed on spread footings or on deep foundations. Historically, the dominant type of semigravity retaining wall is the conventional cast-in-place concrete cantilever structure. Alternatively, various kinds of pre-cast concrete walls are available in the market, which walls are constructed on cast-in-place footings. Cantilever semi-gravity retaining walls may be very reliant on the dead weight of the soil mass that rests on the section of the foundation footing that extends back beyond the wall""s stem, while also developing the necessary structural resistance. An example of the necessary structural resistance would be the wall""s moment and shear capacity at the base of the stem.
Nongravity walls derive their restraining capability through lateral resistance. This lateral resistance may be mobilized in a number of ways. For example, the continuation of vertical structural elements down to competent soils, or the use of ground anchor retainers directly delivering point resistance to the retaining structure. Examples of externally stabilizing nongravity systems are embedded cantilevering wall elements, sheet piles, drilled shafts, or slurry walls. A second group of nongravity walls includes the first listing of embedded walls but have additional restraint via utilizing multiple ground anchor retainers.
Where, for example, there is a need to arrest the creep movement of a slope, nongravity systems may be employed in the form of dowel piles or caissons, to internally stabilize the soil mass. It should be noted that required equilibrating forces may be developed via the use of reaction members which develop point-reaction-forces. (Consider the reactions to a truss, which truss transfers moment to its support). That is, the structural elements delivering resistance to the retaining wall structure overall may have so little moment (and shear) resisting capacity, if any, that the equilibrating set of forces are established via point-acting reaction forces. For example, an arrangement of elements for such a system, may consist of a set of vertical (or near vertical) piles, a set of (near) vertical ground anchors and, finally, a set of (near) horizontal ground anchors. In this case, the piles would take up compression loads, the (near) vertical ground anchors would provide a (predominantly) downward reaction, which would act in concert with the piles"" upward reaction to provide moment resistance to the base foundation. The (near) horizontal ground anchors, placed appropriately at the foundation beam/pile cap level, would resist the net xe2x80x9cshearxe2x80x9d forces from the retaining wall structure that would cause the foundation element to translate.
An example of a retaining wall is shown, for example, in U.S. Pat. No. 2,149,957 (xe2x80x9cthe Dawson patentxe2x80x9d). The wall of the Dawson patent utilizes stretchers and headers to construct a retaining wall. Dawson further discloses xe2x80x9cpositive tensile anchorage.xe2x80x9d Such xe2x80x9cpositive tensile anchoragexe2x80x9d refers to the construction of the individual elements and has no impact on the primary behavior of the system disclosed in the Dawson patent. Moreover, the wall of the Dawson patent does not pre-stress header assemblies through post-tensioning. Further, the Dawson patent does not disclose vertically disposed passive reinforcement through the header assemblies.
Retaining wall systems, such as those shown in the Dawson patent, often do not provide an optimal solution for retaining or supporting an applied load. The design of conventional retaining wall systems may result in constructibility problems, resulting in longer construction periods, higher cost, and more extensive use of the surrounding land. Thus there is a need in the art for a retaining wall system that provides an improved solution for retaining or supporting an applied load and overcomes the limitations of constructibility problems with existing systems. There is a further need in the art for a retaining wall system that is modular and adaptable to a wide variety of construction needs.
The present invention solves the problems with, and overcomes the disadvantages of conventional retaining wall systems. Accordingly, the present invention provides a system and method for constructing a pre-stressed modular construction for supporting or retaining an applied load. The retaining wall systems of the present invention are specifically designed to provide the owner, architect, engineer, and constructor with retaining wall solutions that most adequately provide for more difficult sites and/or increased performance expectations.
The present invention relates to a system and method for constructing a pre-stressed modular construction for supporting or retaining an applied load. In particular, the present invention relates to a system and method for constructing pre-stressed modular retaining walls. In one aspect of the present invention, a system for constructing a pre-stressed modular construction for retaining or supporting an applied load is provided. The system comprises a header stack, wherein the header stack is comprised of a plurality of header units; and an active reinforcement element configured to cooperate with the header stack so that post-tensioning the active reinforcement element imparts a corresponding pre-stressing force into the header stack. In one embodiment of the invention, the header units that make up the header stack comprise a center element having a top face, and a bottom face; a first end element disposed at one end of said center element; and a second end element disposed at another end of said center element.
The system may comprise active reinforcement elements disposed external to the header stack. In such a configuration, there may be passive reinforcement elements disposed internal to the header stack. Additionally, active reinforcement elements may be disposed internal to the header stack.
In another aspect of the system, the header units that make up the header stack comprise a top face and a bottom face; a base element having a first end and a second end; a head element having a first end and a second end; and a pair of side elements extending between each of the first end and the second end of the base element and the head element. The system further comprises a structural member for coupling two or more header stacks and a complementary structural element disposed between two header units and extending between two or more header stacks.
In another aspect of the invention, a pre-stressed modular construction for retaining or supporting an applied load is provided. The construction comprises a plurality of header stacks, wherein each of the header stacks comprises a plurality of header units; and a plurality of active reinforcement elements configured to cooperate with at least one of the header stacks so that post-tensioning the active reinforcement element imparts a corresponding pre-stressing force into the header stack. There are a plurality of structural members, wherein each of the structural members is coupled to at least one of the header stacks. In an exemplary embodiment of the construction, the header units that make up the header stack comprise a center element having a top face, and a bottom face; a first end element disposed at one end of the center element; and a second end element disposed at another end of the center element.
In another aspect of the pre-stressed modular construction, the header units that make up the header stack comprise a top face and a bottom face; a base element having a first end and a second end; a head element having a first end and a second end; and a pair of side elements extending between each of the first end and the second end of the base element and the head element. The construction further comprises a structural member for coupling two or more header stacks and a complementary structural element disposed between two header units and extending between two or more header stacks.
In a further aspect of the invention, a pre-stressed modular construction for retaining or supporting an applied load is provided. The pre-stressed modular construction preferably comprises at least two header stacks, each of the header stacks being comprised of a plurality of stacked header units. There is also preferably at least one pre-stressing tendon for each of the header stacks, with each pre-stressing tendon being configured to cooperate with its header stack so that post-tensioning the pre-stressing tendon prior to application of the applied load imparts a corresponding pre-stressing force into its header stack at at least one lock-off point. There is also a structural member coupled to the at least two header stacks. The pre-stressed modular construction further preferably comprises a tieback transfer beam disposed between two of the header units and extends between the at least two header stacks. There is also a ground anchor coupled to the tieback transfer beam. The structural member can be a concrete stretcher, a pre-cast concrete panel, a cast-in-place concrete panel, a cast-in-place concrete arch, or shotcrete.
In another aspect of the invention, a method of fabricating a pre-stressed modular construction for retaining or supporting an applied load is provided. The method comprises providing a foundation for the construction; constructing a plurality of header stacks on the foundation, with each header stack being comprised of a plurality of header units; coupling an active reinforcement element to each header stack; and post-tensioning the active reinforcement element such that it imparts a corresponding pre-stressing force into the header stack. The constructing step comprises stacking a plurality of header units. The coupling step comprises pre-positioning the active reinforcement element in the foundation; feeding the header units over the active reinforcement element, the active reinforcement element passing through passthrough ducts in the header units; and securing the active reinforcement element to the header stack. In a configuration where external active reinforcement elements are used, the active reinforcement elements may be locked off in a variety of ways. The active reinforcement elements may be locked off at external coupling devices coupled to the header stack, or locked off at a complementary structural element.
In a further aspect of the invention, a method of fabricating a pre-stressed modular construction for retaining or supporting an applied load is provided comprising the steps of suspending a plurality of header units; casting a foundation beneath the header units; constructing a plurality of header stacks on the cast foundation, wherein each header stack is adjacent one of the plurality of suspended header units; coupling an active reinforcement element to the header stack; and post-tensioning the active reinforcement element such that it imparts a corresponding pre-stressing force into the header stack.
In a further aspect of the present invention, a method of fabricating a pre-stressed modular construction for retaining or supporting an applied load is provided. The method comprises the steps of providing a foundation for the construction; constructing a plurality of header stacks on the foundation, wherein each header stack comprises a plurality of header units; coupling an active reinforcement element to each header stack; post-tensioning the active reinforcement element such that it imparts a corresponding pre-stressing force into at least one of the header stacks; providing additional header units to at least one of the header stacks; and repeating the step of post-tensioning after application of another portion of the applied load. In a still further aspect of the present invention, a method of fabricating a pre-stressed modular construction for retaining or supporting an applied load is provided. The method comprises the steps of providing a foundation for the construction; constructing a plurality of header stacks on the foundation, wherein each header stack comprises a plurality of header units; coupling an active reinforcement element to each header stack; imparting a portion of the applied load to the modular construction; post-tensioning the active reinforcement element such that it imparts a corresponding pre-stressing force into at least one of the header stacks; providing additional header units to at least one of the header stacks; and repeating the step of post-tensioning after application of another portion of the applied load.
Features and Advantages
An advantage of the present system is that structural pre-stressing may be sequentially modified, most typically increased, as the soil loading on the retaining wall changes.
Another advantage of the present system is that retaining wall (vertical) sections may be given sufficient and/or final pre-stress so as to allow for the construction of other structural members. If necessary, this could all take place before the soil loads are placed on the wall.
A further advantage of the present system is that the retaining wall structure may be stressed so as to always possess xe2x80x9cresidualxe2x80x9d, or xe2x80x9cnetxe2x80x9d, compressive stress on the xe2x80x9ctensionxe2x80x9d side of any given header stack cross-section. This latter characteristic would be called on in environmentally hostile situations. For example, environmentally hostile situations may exist where naturally aggressive minerals are present in the ground water in contact with, or in close proximity to, the retaining wall, or where the retaining wall is a sea wall.
An advantage of the system of the present invention is ready availability. Short period cyclic casting of standardized structural modules assures that structural components are produced in sufficient quantities to satisfy fast track construction schedules.
A further advantage of the system of the present invention is superior quality control. Plant-cast pre-cast concrete components are manufactured under optimum conditions of forming, fabrication and placement of the reinforcement, inclusion of pre-stressing passthrough ducts and other embedded items and features. The optimally controlled placement and compaction of low slump concrete having optimized mix design and control, along with favorable curing conditions, typically not achievable on site, further significantly increase the in-service performance of these elements.
Yet another advantage of the system of the present invention, for retaining wall construction possessing a given structural capacity, is reduced construction depth. High performance concrete is easily achievable. For any given loading conditions, via the correct selection of (sub)group of components, the retaining structure depth may be minimized, a significant advantage where space is at a premium.
Another advantage of the system of the present invention is its high load-resisting capacity. For a given set of spatial restrictions and/or for a given volume of materials used, pre-cast pre-stressed concrete offers greater structural strength and rigidity. These attributes become very significant in many applications.
A further advantage of the system of the present invention is its durability. Pre-cast concrete, in particular high-performance pre-cast concrete, is exceptionally resistant to weathering, abrasion, impact and corrosion. The resulting structures have great resistance to the deleterious effects found in hostile environments.
Yet another advantage of the system of the present invention is its long economic life. The reliability of currently available pre-stressing systems and the durability characteristics of the pre-cast elements allow for the economic construction of very-long-life retaining and/or support structures. Pre-stressing reduces or, if required, completely eliminates tension cracks, and thereby guarantees the integrity of the concrete and the protection of the embedded steel elements.
Another advantage of the system of the present invention is derived from the use of architectural concrete. The process of pre-casting concrete components, for example, the pre-cast panels that may be used with certain embodiments of the present invention, lends itself to the sculpturing of these exposed elements, and the consequent enhanced appearance of the final structure.
Still another advantage of the system of the present invention is the flexibility of construction sequence. The application of pre-stress, in particular the staged and/or sequenced application of pre-stress, to the assemblies of pre-cast concrete modules in these systems allows for sequenced construction without re-setup penalties.
Another advantage of the system of the present invention is the control of shrinkage and creep, and the consequent effects of same, which control can essentially be xe2x80x9cdialed up.xe2x80x9d In this regard, the ready quality control of concrete products, that are manufactured via plant-cast pre-casting, affords greater accuracy in the determination of anticipated shrinkage and creep. With knowledge of the characteristics of pre-stressing components and the concrete characteristics of the various modules, along with the control of the pre-stressing stress magnitudes and distributions, the shrinkage and creep may be accurately predetermined.
Another advantage of the system of the present invention is the reduction or complete elimination of site formwork. Certain embodiments of the invention, as built above foundation level, are constructed entirely independent of cast-in-place concrete.
A further advantage of the present invention is its speed of construction. The fact that all embodiments can employ pre-cast header modules, used to form the header stacks, and some can be completely comprised of pre-cast elements, contributes significantly to the guaranteed speed of erection. One of the principal aims of these systems is to provide retaining wall and/or support structural systems that, not only provide high capacity, but may be erected with great rapidity.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned in practice of the invention.